1. Field of the Invention
The present invention relates to a data storage device, such as a hard disk apparatus. The present invention also relates to a hard disk apparatus, which finds use as a main or auxiliary storage device of a computer system.
2. Description of the Related Art
Hard disk apparatuses generally have a clean chamber defined by a base member and a cover member, and one or more hard disks accommodated in the clean chamber so as to be accessible by a magnetic head supported by a head arm.
More specifically, at least a rotor hub of a spindle motor is rotatably received in the clean chamber, and one or more hard disks are mounted on the rotor hub. The clean chamber additionally accommodates a head assembly which includes a voice coil motor. The voice coil motor serves to rotationally drive the head arm, such that the magnetic head on an end of the arm traces the surface of each disk to read and write data.
In the hard disk apparatus of the type described above, the air inside the clean chamber must be maintained at a high degree of cleanness in order to enhance the recording density and to prevent any crush attributable to jamming of dust particles between the head and the disk surface. This requires not only that the interior of the clean chamber be shielded from the ambient air to prevent invasion by dust, but also that the generation of dust particles, or so-called contaminants, inside the clean chamber be suppressed.
Stainless steels, which are resistant to rusting as compared with ordinary steels or structural steels, are most commonly used as the materials for parts inside the clean chamber, such as the shaft and the rotor hub of the spindle motor, the shaft and arm of the head assembly and other elements, such as screws. The use of such stainless steels is intended mainly to satisfy the above-described requirements.
At the same time, a high degree of dimensional precision is required for parts of a precision machine or device, such as a hard disk apparatus. This, in turn, requires a high degree of workability of the material to be used. Among the various types of working techniques which are available, cutting, i.e., machining, is most frequently employed. Free cutting steels, which contain various free cutting elements serving to improve cutting characteristic, i.e., machinability, are therefore finding broadened use.
Although stainless steel in general is hard to machine, machinability can be improved by the addition thereto of free cutting elements, such as sulfur (S), tellurium (Te), selenium (Se), etc. Stainless steel containing sulfur as a free cutting element at a content of 0.25 percent by mass or greater is commonly used because of its moderate cost and good machinability. In this type of steel, sulfur exists primarily in the form of manganese sulfide (MnS). The manganese-to-sulfur content ratio Mn/S is usually set at about 4, in consideration of stability in production and ease of machining.
The above-described stainless steel is, thus, rich in sulfur, which is added for the purpose of improving machinability, and is present primarily in the form of manganese sulfide. There is, however, a risk that manganese sulfide will generate a corrosive hydrogen sulfide (H.sub.2 S) gas upon reaction with the moisture in atmospheric air. For this reason, ordinary stainless steels having an Mn/S ratio of about 4 are not suitable for use as the material for parts of a hard disk apparatus in a hermetic clean chamber.
In contrast, stainless steels having an Mn/S ratio not greater than 1.8 are substantially free of generation of sulfur-containing gases. Such smaller value of the Mn/S ratio, i.e., a smaller Mn content, reduces the tendency of formation of the chemically active manganese sulfide. Instead, the formation of chromium sulfide (CrS), which is more stable than manganese sulfide, is enhanced.
Stainless steels having an Mn/S ratio not greater than 1.8, however, tend to rust. At the same time, they exhibit a higher chemical stability in wet environments. In addition, these stainless steels are liable to generate particles containing sulfur, which is intended to improve machinability. This poses a risk of contamination by corrosive particles.
A stainless steel containing 0.25 percent by mass or greater of sulfur as a free cutting element at a manganese-to-sulfur ratio Mn/S of around 4 may be subjected to a heat treatment, e.g., baking. This serves to forcibly release sulfur gas, thereby suppressing contamination. In addition, such a stainless steel may be passivation-treated to attain improved corrosion resistance while allowing removal of particles. A heat treatment as described above may be conducted after the passivation treatment. Processes including such treatments, however, are not recommended from the view points of yield and equipment cost.
It is also possible to employ a free cutting stainless steel having a sulfur content of 0.25 percent by mass or greater at a manganese-to-sulfur ratio Mn/S of 1.8 or less. This free cutting stainless steel, however, also poses a problem when subjected to an acid cleaning conducted with an acid solution having an acid concentration exceeding 15 percent by volume. In this regard, it permits the dissolution of free cutting grains rich in chemically stable CrS, thereby allowing the generation of H.sub.2 S.
Under these circumstances, it is an object of the present invention to provide a hard disk apparatus which suppresses the generation of sulfur-containing corrosive gas from stainless steel in the clean chamber. This prevents a reduction in resistance to corrosion while preventing particle contamination caused by matter which may be freed from surfaces of the parts constructed of the stainless steel.
Other objects and advantages of the present invention will become apparent to one of ordinary skill in the art upon review of the specification, drawings and claims appended hereto.